This invention relates to the measurement of the thickness of a film on a substrate to determine if the thickness is within prescribed maximum and minimum limits. One application of this invention is to measure the thickness of a protective plastic film on a sheet of metal or other self-sustaining material used in the can making industry.
It is old to measure film thickness upon a substrate by passing a beam of light therethrough and measuring the absorption of the beam in passing through the film, which absorption is a function of film thickness. If the substrate is moving at a high speed, measurements present a problem due to the bounce of the sheet, or due to the formation of creases, particularly if the sheet is quite thin and is subject to creasing. In the case of an opaque substrate that has reflecting powers, it is common to measure the film thickness by measuring the light absorption as a beam of light passes through the film and is reflected from the base or substrate to pass through the film again. Flexing of the substrate causes uncontrolled changes in the direction of the reflected light beam, and may drastically affect the measuring action. It is an object of this invention to provide a measuring apparatus of the above type wherein normal flexing or bouncing of the moving material under measurement, or the normal light dispersion within the material being measured, will not substantially affect the measurements. This is accomplished, in accordance with a preferred embodiment of this invention, by providing a light source which is a black-body radiator that is maintained at a regulated temperature so that the infra-red light rays emitted thereby are of substantially known wave lengths, and locating that radiator in a position such that it substantially encircles the area of the film that is being scanned at any particular instant in time whereby all observations of said area regardless of the angle of normal flexing of the material will still constitute an observation of some area of the radiator. The radiator is maintained at a uniform light intensity throughout its entire area. In one preferred embodiment, the black-body radiator is an isothermal cavity that is substantially hemispherical in shape, and the portion of the moving film that is being examined at any instant in time is a portion that is located approximately at the center of the hemisphere. A pyrometer is responsive to the infra-red or other rays radiated by the source. An infra-red pyrometer is useful for this measurement since it must operate on a wave length region of high absorption by the subject material of the film. Since the pyrometer responds not only to the radiation from the source passing through the film, but also the infra-red radiation from the surface of the film itself, it is important that the source radiation be of a high intensity compared to the radiation from the film surface itself. Therefore, the source radiation is maintained at a high temperature, say at the order of 500.degree.F (or more) when the film temperature is room temperature. The film whose thickness is being measured is at a temperature at the order of 125.degree.F or less. In the range of wave lengths used, the infra-red radiation is an exponential function of the temperature. That function is generally above the fourth power. Therefore, the infra-red radiation of the film being measured is a negligible part of the infra-red radiation of the black-body radiator.
The attainment of the above and further objects of this invention will be apparent from the following specification taken in conjunction with the accompanying drawing forming a part thereof.